Polymer coatings which can be cured by exposure to ultraviolet (UV) or other radiation have achieved a high degree of practical utility since they offer many specific advantages. UV curing systems can be comprised of 100% reactive components, thereby avoiding the need for solvents and their safety and environmental hazards. UV curing is faster and more cost effective than alternative processes, such as thermal curing. Further, curing compositions can readily be tailored to meet specific application requirements since available functional monomers and oligomers cover a wide range of properties.
Photocuring coating compositions are usually comprised of three components: the monomer, a photoinitiator, and (optionally) additives. The monomer is an unsaturated or cyclic organic compound which can undergo polymerization. It can be a single compound or a mixture of compounds. Typical monomers employed in photocuring coating compositions include styrene-unsaturated polyesters, acrylates, thiol-enes, allyl ethers, vinyl ethers, epoxides, and the like. The monomers are preferably bi- or multi-functional, to form a highly cross-linked network providing maximum strength and stability.
The photoinitiators are photosensitive compounds which absorb UV radiation and produce activated species--typically free radicals (for styrene, acrylate and allyl monomers) or cationic in nature (for vinyl ethers and epoxides)--to initiate polymerization or cross-linking. These photoinitiators can be used as a single compound or a mixture of different compounds, to meet desired curing characteristics.
The additives include the usual pigments, dyes, inhibitors to prevent polymerization during storage, stabilizers to reserve optimum properties and prolong service life, regulators to adjust flow characteristics, and the like.
Acrylic coatings, including those based on mono- and multi-functional acrylates, methacrylates, and acrylic oligomers derived from epoxides, polyesters, and polyurethanes combine the advantages of superior film properties (clarity, strength, adhesion, gloss, etc.); adjustable flow properties for easy application; and rapid curing. Acrylic coatings based on fluoroalkyl acrylic ester monomers (herein sometimes referred to as "F-acrylates") possess highly desirable properties such as high thermal and chemical stability, low surface energy, low friction, and low refractive index. Such advantageous properties are dependent on fluorine content; they tend to improve with increasing fluorine content. For example the refractive index of amorphous fluoroacrylic polymers decreases with increasing length and number of the fluoroalkyl chains. Unfortunately, the photoinitiators conventionally employed for polymerizing the usual acrylic monomers are not miscible with some highly fluorinated acrylate monomers, and there are no ready means for photocuring such acrylate monomers. UV curing of such fluorinated acrylate monomers without the aid of a photoinitiator, while feasible, gives less than optimum properties, is time consuming and expensive.
Accordingly, it is an object of this invention to provide new photoinitiators which are compatible with highly fluorinated acrylate monomers.